A publication of
CHEMICAL ENGINEERING TRANSACTIONS
VOL. 35, 2013
The Italian Association
of Chemical Engineering
www.aidic.it/cet
Guest Editors: Petar Varbanov, Jiří Klemeš, Panos Seferlis, Athanasios I. Papadopoulos, Spyros Voutetakis
Copyright © 2013, AIDIC Servizi S.r.l.,
DOI: 10.3303/CET1335193
ISBN 978-88-95608-26-6; ISSN 1974-9791
Testing of Zeolite and Kaolin for Preventing Ash Sintering
and Fouling during Biomass Combustion
Liang Wang*, Michael Becidan, Øyvind Skreiberg
SINTEF Energy Research, Sem Sælands vei 11, Trondheim, Norway
[email protected]
Formation and release of potassium chloride (KCl) during biomass combustion cause ash sintering and
deposition during combustion of biomass fuels. In the present work, two mineral additives, zeolite 24A and
kaolin, were tested in terms of their capacity for capturing KCl. This was done experimentally using a
thermogravimetric method. The effects of zeolite 24A and kaolin on sintering behaviours of barley straw
ash were also studied by a combination of laboratory sintering test, SEM-EDX and X-ray diffraction
analyses. It was found that both zeolite 24A and kaolin have high capacities to capture KCl at elevated
temperatures. Instead of evaporating into gas phase more than 50 wt% and 40 wt% of the tested KCl were
retained in zeolite 24A and kaolin at 1,000 °C, respectively. The amount of KCl captured by zeolite 24A
and kaolin decreased with increasing temperatures. Compared to kaolin, zeolite 24A was more effective
for capturing KCl at the tested conditions. Both zeolite 24A and kaolin can react with KCl to form different
potassium aluminium silicates. It indicates that chemical reactions play an important role in the overall
capturing process. The barley straw ash melted severely at 1,000 °C due to formation and melting of
potassium silicates. Addition of kaolin and zeolite 24A significantly reduced the sintering tendency of the
barley straw ash. Together with SEM-EDX analysis, identification of high temperature melting potassium
aluminium silicates by XRD partly explains the improved sintering behaviours of the ash-additive mixtures.
1. Introduction
Due to the development of bioenergy and the limited availability of virgin woody fuels, biomass materials
such as residues from the agricultural sector are entering the market for heat and power production.
Compared to woody biomass, the agricultural residues contain a high amount of ash forming matters. Ash
forming elements such as K, P, Si and Cl are abundant in agricultural residues. These inorganic elements
in biomass fuels are nutrients needed for plants growth and often enhanced as a result of utilization of
fertilizers (Boström et al., 2011). The combustion of agricultural residues is often challenged by ash related
operational problems, including ash sintering, fouling deposits and high temperature corrosion of heat
transfer components. Potassium plays a critical role in initiating and boosting these problems during the
combustion process (Hupa, 2011). During combustion, part of the potassium in agricultural residues
readily releases as KCl gases causing fouling deposits on heat transfer tube surfaces. Potassium retained
in solid combustion residues may react with silicon to form low temperature melting potassium silicates
initiating ash sintering and slagging (Wang et al., 2012a). Utilization of additives have been investigated for
binding and/or abating formation of problematic potassium chlorides during biomass combustion As a
representative of aluminium silicates based additives, kaolin can bind potassium chloride in aluminium
silicates and releases Cl in the form of HCl (Steenari and Lindqvist, 1998). Recent literature reports that
zeolite, as a member of the aluminium silicates additives, can reduce fouling deposits during combustion
of high potassium biomass fuels (Elled et al., 2010). In addition, zeolite is mainly used for washing powder
production, and largely ends up in sewage sludge (De Filippis et al., 2013). Interactions of zeolites in the
sewage sludge with problematic potassium species were also observed (Pettersson et al., 2009), opening
the possibility of using sewage sludge as additive. These interactions reduced the concentration of KCl in
flue gas and formation of fouling deposits on the sampling probe consequently. However, studies about
Please cite this article as: Wang L., Becidan M., Skreiberg O., 2013, Testing of zeolite and kaolin for preventing ash sintering and fouling
during biomass combustion, Chemical Engineering Transactions, 35, 1159-1164 DOI:10.3303/CET1335193
1159
the capturing of KCl by zeolite are rare. The purpose of this work is to characterize the capacity of zeolite
to adsorb KCl by a thermogravimetric method. The possible mechanisms of KCl adsorption by zeolite were
investigated. In addition, the effects of zeolite on sintering behaviours of ash from one agricultural residue,
barley straw, were investigated. Kaolin was also tested in the current work for comparison purpose.
2. Experimental section
Barley straw was used in this study, and was combusted at 550 °C for 10 h to produce reference ash. Two
additives, zeolite 24A and kaolin, were tested. With proven alkali getter capacity, the kaolin was used as a
reference additive in this work. The zeolite 24A tested in this study is a widely used detergent zeolite with
the chemical formula (Na2O·Al2O3·2SiO2)2H2O, and has a particle size smaller than 2 μm. The particle
size of kaolin powder is smaller than 10 μm. The chemical compositions of barley straw ash, zeolite 24A
and kaolin were measured by X-ray fluorescence (XRF) and are presented as oxides in Table 1. Major
mineral phases in barley straw ash, zeolite 24A and kaolin were analyzed by an X-ray diffractometry
(XRD) spectrometer (Bruker D8 focus).
It is well known that KCl is directly responsible for biomass ash related problems. One main objective of
this study is to test the capacity of zeolite 24A and kaolin to capture KCl. This was achieved by performing
a simple thermogravimetric method. In the tests, one additive was thoroughly mixed with solid KCl
particles in a platinum crucible in a molar ratio 1:2. This molar ratio represents the amount of the additive
that could react with added KCl according anticipated stoichiometric reactions. KCl will be mostly
vaporized after a long enough time at a temperature higher than 850 °C (Steenari and Lindqvist, 1998).
The sample mixtures were then heated at 900 °C and 1,000 °C for 1 h under oxidizing atmosphere in a
muffle furnace. During the heating, the KCl may vaporize and the additive retains the KCl through physical
adsorption and/or chemical reactions. After heating, the residues of the sample mixtures were weighed
while they still were warm to avoid adsorption of moisture from the ambient air. The weight differences
between initial and final mass of the sample mixtures were calculated, with subtraction of weight loss of
raw additives at the same condition. The final results were considered as amount of KCl captured by one
additive, which were described as retention mass fraction of KCl initially blended in the additive.
To investigate the effects of additives on barley straw ash sintering behaviors, the reference fuel (550 °C)
and ash-additive mixtures were heated at 1000 °C for 1 h. The stoichiometric amount of additive to
theoretically capture all potassium in the barley straw ash was added. Therefore, 4 % (w/w) additive was
added to the reference barley straw ash. After cooling down to room temperature, sintering behaviors of
residues from reference ash and ash-additive mixtures were visually evaluated. Then the solid residues
were collected for further analyses. Part of the collected solid residues was embedded in resin and crosssectioned for SEM-EDX analysis. The SEM was operated in a backscattered electron mode for illustrating
distribution of detected elements in a scanned area. EDX semi-quantitative spot/area analyses were
carried out for interesting areas to get more detailed microchemistry information. In addition, residues from
reference ash and ash-additive mixtures after heating treatment were also grinded into powder and
analyzed by XRD.
3. Results and discussion
3.1 Fuel and additives
The elemental compositions and crystalline compounds of the barley straw ash and the two additives are
listed in Table 1 and Table 2. Abundance of K, Cl, Si and P in the barley straw ash strongly indicates the
existence of K containing chlorides, silicates and phosphates. This assumption was proved by XRD
analysis results. As shown in Table 2, KCl and K 2SO4 were detected together with KCa(PO3)3 and
K4Ca3SiO4 in the barley straw ash.
Table 1: Chemical compositions of barley straw ash and additives used in this study
a
Ash analysis (wt %. db ) K2O
c
Barley straw ash
33.64
Kaolin
0.2
Zeolite 24A
0.03
SiO2
27.50
46.12
34.26
CaO
16.43
0.07
b
n
P2O5
8.07
b
n
b
n
Al2O3
0.18
34.08
26.35
MgO
2.91
0.16
b
n
Na2O
0.38
0.04
11.74
Fe2O3
1.73
0.87
0.01
SO3
4.90
0.06
0.01
Cl
3.28
b
n
b
n
a: dry basis; b: not detected; c: ash produced at 550 °C
1160
Table 2: Major mineral phases observed in barley straw ash (550 °C) and raw additives
Major mineral phases
Barley straw ash
Kaolin
Zeolite Doucil 24A
KCl, K2SO4, KCa(PO3)3, K4Ca3SiO4, SiO2, CaCO3
Al2Si2O5(OH)4, SiO2
(Na2O)(Al2O3)(SiO2)2·5H2O
The KCl and K2SO4 have rather low melting temperatures, about 770 and 850 °C, respectively. Melting of
these potassium salts is one of the dominating mechanisms causing sintering of ash during biomass
combustion. Table 2 shows that kaolinite (Al2Si2O5(OH)4) is a major component of the kaolin used in the
current work. Zeolite 24A contains (Na2O)(Al2O3)(SiO2)2·5H2O as a major compound.
3.2 Adsorption of KCl by additives
Retention of KCl by the two additives at 900°C and 1,000 °C are shown in Figure 1. It can be seen that
both zeolite 24A and kaolin have high capacities to capture KCl. Even at 1000 °C, about 50 % and 40 % of
the added solid KCl were retained in zeolite 24A and kaolin, respectively. However, less KCl was captured
by the two additives at 1,000 °C compared to 900 °C. The capturing of KCl by additive particles involves
physical and chemical adsorption (Kyi et al., 1999). Decreased KCl capturing capacities of the two
additives at a higher temperature are probably due to loss of reactive surface areas on the additive
particles, which restrains physical adsorption and chemical reactions of additive particles with the KCl
vapor (Wang et al., 2012b). In addition, the porosity of the additive particles decreases at a higher
temperature. It will limit the amount of KCl vapor diffusing into additive microstructure and reacting with
active compounds in the additive (Tran et al., 2005). On the other hand, the chemical incorporation of KCl
into additive particles may become difficult due to the transformation of main active compounds such as
metakaolinite into rather inert new phases at elevated temperatures (Zheng et al., 2008).
Figure 1 shows that, compared to kaolin, more KCl was retained in zeolite 24A at the two tested
temperatures. It is probably related to the different microstructure of kaolin and zeolite 24A. Zeolite 24A
has a channel pore structure and intracrystalline voids. Vaporized KCl may diffuse into the channels and
voids followed by reactions with aluminum silicates contained in zeolite 24A. At a certain high temperature,
the original structure of zeolite 24A may partially collapse and occlude all KCl vapor into the structure. On
the other hand, KCl easily desorbs from kaolin particles with a flake like structure, when they are heated at
high temperature or for long enough time. In brief, zeolite 24A has a microstructure that is more favorable
for capturing KCl vapor.
The mineral phases in the solid residues from additive-KCl mixtures were analyzed by XRD. KAlSiO4 was
observed in residues from the kaolin-KCl mixture, which has also been observed as a main product from
reactions of kaolin with KCl in other study (Steenari and Lindqvist, 1998). Formation of KAlSiO4 was also
observed from the zeolite 24A-KCl mixture sintered at 1000 °C. In addition, the mineral phases KAlSi3O8,
KNa3Al4Si4O16, (K,Na)Si3AlO8 and KNa3(Al,Si)4O8 were identified from the heated zeolite 24A-KCl
mixtures. Formation of these potassium aluminum silicates indicates that zeolite is chemically active to
react with KCl via different reaction paths. These chemical reactions are normally irreversible and
favorable to bind KCl vapor, which can abate KCl initiated ash sintering and fouling deposits consequently.
Figure 1: Retention of KCl within zeolite 24A and kaolin at different temperatures
1161
3.3 Effect of additives on sintering behaviour of barley straw ash
The effects of the additives on sintering behaviour of the barley straw ash were evaluated by performing
laboratory scale sintering tests. The reference barley straw ash and mixtures with additives were heated at
1,000 °C for 1 h. After the heating treatment, the barley straw ash melted completely on the bottom of the
crucible. Broken bubbles were observed from the melted ash residues. It implies that the ash passed
through an intensive melting stage and behaved as liquid. With addition of additive, severe melting of the
barley straw ash was mitigated. The residues from the mixture of barley straw ash and kaolin heated at
1000 °C are slightly sintered with formation of fine aggregates that are easily broken. However, the residue
from the mixture of the barley straw ash and zeolite 24A was sintered with the formation of large ash
aggregates. Part of the residues was melted and formed a solid layer on top of the residues after the
heating treatment. The sintering tests in this study showed that kaolin is more effective than zeolite 24A for
reducing sintering tendency of the barley straw ash. Similar findings have been reported in a previous
study. Kaolin has the ability to eliminate sintering of ash from wheat straw in the temperature range 7001,000 °C. However, as zeolite 24A was added, wheat straw ash partially agglomerated as the heating
temperature became higher than 900 °C (Wang et al., 2012b).
The residues from barley straw ash and corresponding mixtures with additives after heating treatment
were analysed by SEM-EDX and XRD. Figure 2 shows representative SEM images taken from crosssectioned barley straw ash sintered at 1000 °C. It can be seen that the barley straw ash has completely
melted, showing continuous phases. Voids with round rims of different sizes are visible in the melted ash.
The voids represent formation of bubbles as the gases (mainly from carbonates) are released from liquid
ash melts. One zone in Figure 2-a marked with a white rectangle was further examined by EDX. It was
found that the chemical compositions in area 1 and 2 are dominated by K and Si, with small amounts of Ca
and P. This element distribution confirms that melted barley straw ash contains mainly potassium silicates.
It is well documented that potassium readily reacts with silica in the biomass ash to form different
potassium silicates (Boström et al., 2011). Some of the potassium silicates have low melting temperature
and may fuse as viscous liquids leading to the bonding of ash grains. Continuous formation and melting of
potassium silicates may result in bridging of ash aggregates and loss of individual particles (Boström et al.,
2011). With a longer combustion time, ash will be melted completely with a homogeneous and continuous
structure as shown in Figure 2. Compared to the mineral phases contained in ash produced at 550 °C,
only trace amounts of silica were observed from barley straw ash sintered at 1,000 °C. Poor crystallinity of
the barley straw ash sintered at 1,000 °C implies that it has melted into amorphous phases that can not be
directly observed by XRD. A broad hump was observed in the XRD spectrum, which confirms presence of
amorphous phases in the barley straw ash.
Figure 3-a shows a typical SEM image of residues from the mixture of barley straw ash and kaolin after
heating at 1,000 °C. Contrary to barley straw ash, the residues from barley straw ash-kaolin mixture shown
in Figure 3-a have a porous structure and no clear melting can be observed. The big grain illustrated in
Figure 3-a is formed due to agglomeration of fine particles with different sizes. As revealed by EDX
analyses, K and Si are still dominating elements in areas 3 and 4 (Table 4). However, the content of Al
detected in the same area is significantly higher than that of melted barley straw ash (area 1 and 2, Figure
2). It suggests that kaolin addition has introduced more Al in the barley straw ash. This will promote the
formation of high temperature melting potassium aluminium silicates instead of low temperature melting
potassium silicates.
2
1
a
b
Figure 2: SEM-EDX analyses of barley straw ash sintered at 1000 °C
1162
4
5
3
6
a
b
Figure 3: SEM-EDX analyses of mixtures of (a) barley straw ash-kaolin and (b) barley straw ash-zeolite
24A sintered at 1000 °C
Formation of potassium aluminium silicates was confirmed by XRD analysis. Kalsilite (KAlSiO4) and leucite
(KAlSi2O6) were observed in the residues from the mixture of the barley straw ash and kaolin. These two
mineral phases have previously been identified as main products from the residues from mixtures of kaolin
and biomass ashes heated at 1,000 °C (Steenari et al., 2009). Kalsilite and leucite have significantly high
melting points over 1,100 °C. Therefore, formation of these two phases is the main cause for the reduction
of barley straw ash sintering tendency with kaolin addition.
Figure 3-b shows that the residues from the sintered mixture of barley straw ash and zeolite 24A contain
both melted ash and agglomerated particles. The former is clearly visible as a smooth and intact surface.
From the melted ash, EDX revealed that K and Si are two dominating elements. Therefore, formation and
melting of potassium silicates is the main reason for fusion of this part of the ash. However, Figure 3-b
shows that part of the ash in the residue also contains fine particles agglomerated together, with loose
structure. Significant amounts of Al together with Si and K were detected (area 6, Figure 3-b). During the
thermal conversion process, the zeolite decomposes and transforms into different silicates and aluminium
silicates at high enough temperature (Aranda et al., 2012). Some of the aluminium silicates can further
react with potassium containing species in the barley straw ash, leading to formation of high temperature
melting potassium aluminium silicates (Pettersson et al., 2009). Therefore, this part of the ash represents
presence of potassium aluminium silicates such as kalsilite (KAlSiO4) and leucite (KAlSi2O6) identified in
the barley straw ash-zeolite 24A mixture. Formation of these high temperature inert phases improved the
sintering behaviours of barley straw ash with zeolite 24A addition.
Table 4: Chemical compositions of barley straw ash and additives used in this study
Element (wt %)
a
Area 1
a
Area 2
b
Area 3
b
Area 4
c
Area 5
c
Area 6
K
29.8
33.1
27.2
25.3
30.3
24.8
Si
45.1
14.8
35.7
37.2
38.6
40.2
Ca
14.2
39.6
9.8
9.2
7.9
3.4
P
3.4
5.4
1.7
2.1
1.8
2.4
Al
0.7
1.0
19.8
21.2
8.2
16.5
Mg
2.3
2.8
2.2
3.1
1.9
4.3
Na
3.2
2.4
2.1
1.2
8.9
7.4
S
0.7
0.4
0.2
0.4
0.3
0.2
Cl
0.2
0.3
1.1
1.2
1.3
1.0
a: barley straw ash sintered at 1000 °C; b: mixture of barley straw ash and kaolin; c: mixture of barley straw ash and
zeolite 24A
Table 5: Major mineral phases observed from barley straw ash with and without additive addition
Major mineral phases
Barley straw ash
amorphous material, trace amount of SiO2, trace amount of CaSiO3
+ Kaolin
SiO2 (cristobalite), KAlSiO4, KAlSi2O6, CaSiO3,
+ Zeolite 24A
KAlSiO4, KAlSi2O6, (Na,K)AlSiO4, Na4Al2Si2O6, trace amounts of SiO2 and CaSiO3
1163
4. Conclusions
The ability of kaolin and zeolite 24A to capture KCl was investigated via a thermogravimetric study. It was
found that a significant fraction of KCl can be retained in the kaolin and zeolite 24A particles at the tested
temperatures. High melting temperature potassium aluminium silicates were observed from the mixtures of
additives and KCl. It indicates that chemical reactions occurred in the overall capturing process. The barley
straw ash severely sintered at 1,000 °C due to formation and melting of potassium silicates. The intensive
sintering of the barley straw ash was reduced through addition of kaolin or zeolite 24A. Kaolin and zeolite
24A reacted with potassium containing compounds in the barley straw ash. High temperature melting
potassium aluminum silicates were formed, which can partially explain improved sintering behaviors of the
barley straw ash. The results from the current work show that both kaolin and zeolite 24A can be used for
mitigating ash fouling and sintering in biomass combustion applications.
5. Acknowledgement
The authors acknowledge the financial support by the Bioenergy Innovation Centre (CenBio), which is
funded by the Research Council of Norway, a large number of industry partners and seven R&D
institutions.
References
Boström D., Skoglund N., Grimm A., Boman C., Öhman, M., Broström M., Backman R., 2011, Ash
Transformation chemistry during combustion of biomass, Energy Fuels, 26(1), 85-93.
Hupa M., 2011, Ash-related issues in fluidized-bed combustion of biomasses: recent research highlights,
Energy Fuels, 26(1), 4-14.
Wang L., Hustad J.E., Skreiberg Ø., Skjevrak G., Grønli M., 2012a, A critical review on additives to reduce
ash related operation problems in biomass combustion Applications, Energy Procedia, 20, 20-29.
Wang L., Becidan M., Skreiberg Ø., 2012b, Sintering behavior of agricultural residues ashes and effects of
additives, Energy Fuels, 26(9), 5917- 5929.
Steenari B.M., Lundberg A., Pettersson H., Wilewska B.M., Andersson D., 2009, Investigation of ash
sintering during combustion of agricultural residues and the effect of additives, Energy Fuels, 23, 56555662.
Steenari B.M., Lindqvist O., 1998, High-temperature reactions of straw ash and the anti-sintering additives
kaolin and dolomite, Biomass and Bioenergy, 14(1), 67-76.
Elled A.L., Davidsson K.O., Åmand L.E., 2010, Sewage sludge as a deposit inhibitor when co-fired with
high potassium fuels, Biomass and Bioenergy, 34(11), 1546-1554.
Pettersson A., Åmand L.E., Steenari B.M., 2009, Chemical fractionation for the characterisation of fly
ashes from co-combustion of biofuels using different methods for alkali reduction, Fuel, 88(9), 17581772.
De Filippis P., Di Palma L., Petrucci E., Scarsella M., Verdone N., 2013, Production and characterization of
adsorbent materials from sewage sludge by pyrolysis, Chemical Engineering Transactions, 32,205-210
Wang L., Skjevrak G., Hustad J.E., Grønli M., 2011, Effects of sewage sludge and marble sludge addition
on slag characteristics during wood waste pellets combustion, Energy Fuels, 25(12), 5775-5785.
Kyi S., Chadwick B.L., 1999, Screening of potential mineral additives for use as fouling preventatives in
Victorian brown coal combustion, Fuel, 78(7), 845-855.
Tran K.Q., Iisa K., Steenari B.M., Lindqvist O., 2005, A kinetic study of gaseous alkali capture by kaolin in
the fixed bed reactor equipped with an alkali detector, Fuel, 84(2-3), 169-175.
Zheng Y., Jensen P.A., Jensen, A.D., 2008, A kinetic study of gaseous potassium capture by coal minerals
in a high temperature fixed-bed reactor, Fuel, 87(15–16), 3304-3312.
Aranda Uson A., Ferreira G., Lopez-Sabiron A.M., Sastresa E.L. and De Guinoa A.S., 2012,
Characterisation and environmental analysis of sewage sludge as secondary fuel for cement
manufacturing. Chemical Engineering Transactions, 29, 457-462.
1164